st3m_gfx: make partial redraws easier

Issue #77 (closed)

Remove in-flight frame bufffers, for a slight performance hit on low complexity full-screen animations. But making partial redraws possible to manage from client code - as well as having a stable framebuffer that can be more easily hijacked from other user code even bypassing ctx.

This also increases the amount of concurrent textures available, and makes it feasible to experiment with automatic partial redraws from ctx.